1. Field of the Invention
The present invention relates to an ink-jet recording method for color images which comprises shooting droplets of inks (recording liquids) and causing them to adhere to a recording medium to form a color image thereupon. More particularly, this invention relates to an ink-jet recording method suitable for reproducing full-color images with a wide variety of colors and with satisfactory density gradation by using inks of different dye concentrations.
2. Description of the Prior Art
The ink-jet recording method is a recording process which comprises ejecting ink droplets through nozzles by a discharge technique such as a technique utilizing electrostatic attraction by applying high voltage, a technique of giving mechanical vibration or displacement to the ink by using piezoelectric elements, and a technique of heating and foaming the ink instantaneously to utilize the foaming pressure, and causing ejected ink droplets to adhere onto a recording medium such as paper. This method is noticed as a recording process which generates less noises and permits high speed recording.
For recording, by means of ink jet recording methods, of color images delicate in shade like those obtained by silver salt photography, density gradation of colors in images have been reproduced so far by using inks of colors, e.g. cyan, magenta, yellow, and black, and controlling the diameter and number (per unit area) of ink dots marked with these inks to mix colors corresponding to the densities of the three primary color components in original images.
However, this recording technique of varying the diameter and number of ink dots cannot reproduce such gradation of original images as to vary continuously and smoothly over the entire area of from low to high image densities, but gives low resolution images or unnatural images.
Various ink-jet recording techniques have been proposed to solve the above problems. Among these techniques, those employing two inks different in dye concentration for each one color are disclosed in Japanese Laid-open patent application No. 156264/1982 and other documents.
In ink-jet recording technique of the above patent application is characterized in that a plurality of inks different in dye concentration are used for each color, the diameter and number of ink dots are varied according to local densities on the original image, and dots of the ink of lower dye concentration of each color are marked on all the picture elements of that color. With this technique, density gradations particularly in low image density areas can be better reproduced for each color of an image than with conventional ink-jet recording techniques.
According to this recording technique, however, physical properties such as viscosity, surface tension, and pH of the plurality of inks of each color vary from ink to ink since the inks of each color are prepared by dissolving a dye at different concentrations in the solvents consisting of the same composition. In particular, the viscosity variations among the inks will cause variations in the diameter of ink dots (under the same ink droplet discharge conditions) and deviations of falling points of ink droplets from the respective targets on recording media. These problems are significant objections to recording good quality images of high resolution and of natural feeling.
On the other hand, ink-jet recorders are required to be capable of furnishing good quality records over a temperature range as wide as possible, since ink-jet recorders are used not necessarily at a definite temperature but at various temperatures. Similar capability is needed for the inks to be used therein, that is, the inks are required to cope sufficiently with the difference between their preparation and service temperatures or with the variation of environmental temperature during service and to secure constantly good recording performance characteristics and storage stability.
Service temperature of inks are generally different from the preparation temperature thereof. Hence, inks of the same color, if different in the temperature dependence of viscosity, will also differ in viscosity at service temperatures which considerably depart from the preparation temperature, even though the inks have been prepared at a definite temperature so as to be given the same viscosity. This will deteriorate the resulting image quality as stated above.